How did John Dalton turn atoms from an old idea into chemistry?

John Dalton turned atoms into chemistry by giving them weights. The idea that matter is made of tiny indivisible particles was old: the word atom comes from the Greek atomos, meaning indivisible. Dalton proposed that each element consists of identical atoms of a characteristic weight, and that compounds form when atoms of different elements combine in fixed whole-number proportions. That made atoms something a chemist could calculate with.

The groundwork was in the numbers. In 1797 Joseph Proust showed that a compound always contains its elements in the same proportions by weight, whatever its source. In 1804 Dalton noticed that when two elements make more than one compound, the amount of one per measure of the other differs by ratios of small whole numbers. The basic units were indivisible as far as he could tell, so he concluded he had discovered the atoms that chemists and philosophers had long hypothesized.

His essential novelty was a method of calculating relative atomic weights, which allows molecular formulas to be assigned to chemical substances. He published a first table of six elements, with hydrogen taken as 1.

There was a catch. No evidence then showed how many atoms of each element make up a molecule, so Dalton used a controversial rule of greatest simplicity: if two elements form only one known compound, such as hydrogen and oxygen in water, its molecule has one atom of each. So he wrote water as HO. Some problems in his method were later corrected by Gay-Lussac and Avogadro: in 1811 Avogadro proposed that equal volumes of any two gases at equal temperature and pressure contain equal numbers of molecules, and gas volumes then gave water its correct formula, H2O, not HO.